1. Field of the Invention
The present invention relates generally to the field of waste to energy systems and more particularly conversion of organic containing solid, liquid and gaseous wastes into uniform and clean burning synthesis gas. The invention also relates to apparatus for the conversion of solid, liquid, and gaseous organic materials, having fuel value, by reactions with oxygen-bearing gases and water, into uniform gaseous fuel called synthesis gas.
2. General Background and State of the Art
The present invention is directed to a reactor vessel in which solid, liquid and gaseous organic wastes such as but not necessarily limited to forestry and agricultural residues, animal wastes, bacterial sludge, sewage sludge, municipal solid waste, food wastes, animal bovine parts, fungal material, industrial solid waste, waste tires, coal washing residue, petroleum coke, oil shale, even coal, peat and lignite, waste oil, industrial liquid wastes, residuals from petroleum refining and volatile organic compounds generated by the industrial processes are transformed into gaseous fuels with maximum conversion efficiency while maintaining resultant synthesis gas free of tar and oil. The organic materials of this type includes fixed carbon, volatile matter and ash.
Moisture is also included in the volatile matter. The objective of the transformation is to obtain essentially complete conversion of carbon and volatile matter into synthesis gas, while leaving only ash as solid residue. This transformation of the organic material takes place by combining these organic materials with steam and oxygen in a high temperature environment. Gas-solid contact, the temperature and the time allocated for gas-solid contact at a given temperature all play a role in the extent of conversion of the organic material introduced into the reactor vessel. Most of the time, the moisture content of the organic feed material is adequate for the transformation reactions. However, the present invention also includes the benefits of introducing additional moisture to produce uniform quality of the synthesis gas from this apparatus. The present invention does not preclude pre-drying of the organic feed material prior to its introduction into the reactor vessel.
The advantages of converting organic material into synthesis gas over directly combusting the organic material are quite significant. Direct combustion of organic materials mentioned above usually results in smoke and discharge of unwarranted polluting compounds to the detriment of human health. Besides, direct combustion results in deposition of tar in the chimneys which poses a fire hazard. In contrast, the synthesis gas, after production and clean-up, contains simple clean burning combustible gases, namely carbon monoxide, hydrogen and some methane along with non-combustible nitrogen, carbon dioxide and water vapor. This synthesis gas is drawn off to a secondary combustion chamber where heat energy is produced with no smoke, pollution, or tar formation. This synthesis gas is also suitable for fuel use for internal combustion engines.
The organic material transformation systems to produce synthesis gas belonging to the category of this invention falls into one of the three classifications, namely (1) updraft reactor, (2) downdraft reactor, and (3) crossdraft reactor. Under all of these categories, stack of solid organic material moves downwardly while reacting with oxygen-bearing gas and steam. In an updraft reactor, the direction of gas is flowing against the flow of solids, in a downdraft, the gas flows downward with the solids, and in crossdraft reactor, the gas flow traverses the descending column of solids. This invention is directed to an improved design for the downdraft reactor to avoid common pitfalls associated with that particular category of the reactor including tar formation, sintering of solid residue and incomplete conversion.
The downdraft system for converting solid organic material into gaseous fuel is not a new concept. This concept has been used for nearly a hundred years. Reed et al (1988) has succinctly summarized the application and operation of the downdraft gasifier.
In the last two decades or so, interest in biomass gasification has picked up as means of producing energy from renewable resources to supplement the foreign imports as well as to develop strategy for distributed generation for reasons of meeting energy security needs. This renewed interest has encouraged development of new and improved methods for making biomass gasification efficient and fuel gas generated from these cleaner in terms of its tar content. Conventionally, the primary focus of the development has been confined to processing relatively dry solid organic materials in the improved apparatus. The prior art has very little mention about ability to transform solid organic material containing moisture in excess of 20% when fed directly to the reactor vessel or the gasifier. The prior art also lacks any mention of simultaneous transformation of organic solid, liquid and gaseous material.
U.S. Pat. No. 4,306,506 teaches a method for high temperature gasification through injecting air in the oxidation zone prior to the downward moving solids passing through the reduction zone in order to reduce the level of tar in the fuel gas generated in the downdraft gasifier. This patent also teaches a method for the construction of the gasifier vessel comprising inner and outer shells to accommodate vessel expansion when subjected to high temperature.
U.S. Pat. No. 4,309,195 discloses gasifier configuration similar to one taught by U.S. Pat. No. 4,306,506 with an added feature of withdrawing moisture from the upper region of the downward draft gasifier and reinserting the same moisture into the oxidation zone.
U.S. Pat. No. 4,452,611 recognizes the need for turbulence during the solid-gas reaction to avoid sintering of material. This patent teaches method and benefit of multiplicity of jets installed within the downward draft gasifier, albeit, all of the jets are located in one plane more or less like a gas distributor in a fluidized bed. In this method, multiple solid discharges are provided.
U.S. Pat. No. 4,659,340 is an example of crossdraft gasifier that operates at pressures greater than atmospheric pressure. This patent also teaches a method of integrating grate clearing blade within the gasifier for the removal of sinters and clinkers.
U.S. Pat. No. 5,157,176 is an example of updraft gasifier which is used for pyrolysis and gasification of chopped rubber tires.
U.S. Pat. No. 6,048,374 is an example of indirectly heated downward draft gasifier which is propelled by the hot flue gas generated from combustion of residual char resulting from extraction of volatile component from solid organic material which is subjected to contact with hot flue gas.
U.S. Pat. No. 6,615,748 discloses a method of combusting a portion of the organic solid material to supply heat to propel pyrolysis and gasification of the balance of the solid material.
U.S. Pat. No. 6,637,206 discloses a method by which the exhaust gas from the combustor is reacted with solid organic material in a downdraft gasifier in the presence of oxygen and water to produce cleanburning fuel to power an engine.
U.S. Pat. No. 6,647,903 teaches the advantages of a stratified downdraft gasifier in which the oxygen-bearing gas is injected at different levels into the downward moving solids to create several zones for gas-solid interaction. This technique reduced the amount of tar in the fuel gas as well as reduced the incidence of sintering of the ash residual inside the gasifier.